This invention relates to an operational control for two-cycle engines and more particularly to an operational control for two-cycle, crankcase compression, diesel engines.
In two-cycle, crankcase compression engines, it has been proposed to improve the performance of the engine by providing either scavenge and/or exhaust control. Scavenge control can be provided, as taught in U.S. Pat. No. 5,251,584 entitled two-cycle engine, issued Oct. 12, 1993 in the name of Akihiko Ohkubo and assigned to the assignee hereof. Basically, the way that scavenge control is obtained as shown in that patent is by controlling the pressure in the crankcase chamber during the operation. By changing the crankcase pressure through the scavenge control system, the amount of scavenging air flow in the engine can be controlled. This is particularly important with diesel engines since those engines do not normally employ a throttle valve in the induction system. In that particular patent, the way the pressure in the crankcase chambers is controlled is by communicating several crankcase chambers that are on differences phases of their cycles with each other under certain engine running conditions.
Exhaust control is taught in U.S. Pat. No. 5,000,131 entitled EXHAUST PORT CONTROL VALVE FOR TWO-STROKE ENGINE, issued Mar. 19, 1991 and also assigned to the assignee hereof. With such an arrangement, a valve is positioned in the exhaust porting and by opening and closing the exhaust control valve, the effective compression ratio and/or cross-sectional area of the flow passage of the exhaust system can be controlled.
In the aforenoted scavenge and exhaust controls, the respective scavenge and exhaust control valves are positioned in response to the actual running conditions of the engine. This control may be in response to maps generated from one or more parameters of the engine running condition. Although these devices work extremely effectively and can significantly improve the performance of the engine, there are some running conditions which offer a basis for improvement.
For example, the condition of starting of the engine is one wherein the scavenge control and exhaust control cannot be done by sensing the engine running conditions. Therefore, these control valves might be in a position during starting operation that would inhibit or delay effective starting of the engine.
It is, therefore, a principal object of the invention to provide an improved control for either or both of a scavenge system and exhaust system of a two-cycle crankcase compression engine to provide optimum positions during starting operation.
In connection with these valves and particularly exhaust control valves when the engine is shut off and then restarted, there may be some difficultly in changing the position of the exhaust control valve. The reason for this is that the exhaust control valve obviously will be quite hot when shut down after long periods of engine running, particularly at high speeds and high loads. Any residual oil in the engine and in proximity to the exhaust control valve may carbonize under this condition and cause the valve to stick. Means have been provided for cycling the exhaust control valve at the time of engine shutdown so as to achieve a self-cleaning operation. See for example, co-pending application Ser. No. 08/173,208 entitled EXHAUST CONTROL VALVE FOR ENGINE, filed Dec. 23, 1993 in the names of Tateuyuki Masuda et al and also assigned to the Assignee hereof. However, even though the valve may be cycled at the engine shutdown condition, upon start up there still may be some deposits that have formed after the engine has undergone the cleaning cycle and make it difficult to appropriately position the exhaust control valve.
It is, therefore, a still further object of this invention to provide an improved cleaning procedure for an exhaust control valve initiated before the engine is started.
Another condition when the maps of engine parameters are not completely accurate for setting either or both of the scavenge control and exhaust control valves is during sudden acceleration conditions. Both the exhaust and scavenge control valves are operated by servomotors such as stepper motors and these motors have finite operational speeds. Thus, if the accelerator is depressed more rapidly than the servomotors can operate, the position of the control valve will lag that of the demand of the operator. Thus, if accelerator pedal position is one of the parameters under which the control valves are operated, then the actual position of the control valve will not coincide with that called for by the instantaneous position of the accelerator control. This can result in deteriorated running performance.
It is, therefore, a still further object of this invention to provide an improved control for the scavenge and/or exhaust system of a two-cycle, crankcase compression engine that is controlled under conditions of extreme acceleration.
In addition to the delay in appropriately positioning the scavenge and/or exhaust control valves, the fuel control for the engine also may be disturbed during rapid acceleration. That is, the fuel control will operate more rapidly than the exhaust and scavenge controls and since the fuel control under steady state conditions is determined by the expected positions of the scavenge and/or exhaust control valves, then the amount of fuel supply during sudden acceleration conditions may not be appropriate because of the fact that the scavenge and/or exhaust control valves may not be in their appropriate positions.
It is, therefore, a still further object of this invention to provide an improved fuel control arrangement for an engine having scavenge and/or exhaust controls and which can accommodate rapid acceleration conditions.
Another condition of the engine which does not lend itself to the map type steady state controls is when operating at idle. Small variations in speed at idle can call for more abrupt changes in the scavenge and exhaust control position.
It is, therefore, a still further object of this invention to provide an improved scavenged and/or exhaust control arrangement for an engine during idle mode.
A final condition where the performance of the engine can be improved through the use of the scavenge and/or exhaust controls is during extreme deceleration or engine braking conditions. Under these conditions, the operator demand for speed of the engine is abruptly reduced and the engine is used as a brake for slowing the speed of the object which the engine drives. For example, with automotive vehicles it is desirable to be able to use the engine as a brake during some types of deceleration.
With a diesel engine, the engine braking operation is accomplished by shutting off the flow of fuel to the engine. However, this has certain disadvantages, particularly when the engine is supplied with a catalytic exhaust treatment system. As is well known, catalytic exhaust treatment systems for engines must be at an appropriate operating temperature, which is generally quite a high temperature, to achieve optimum efficiency. However, with a diesel engine undergoing engine braking, the shutoff of fuel to the engine causes the engine to operate as a pump and pump large amounts of unheated or cool atmospheric air into the exhaust system. This can obviously deteriorate the performance of the catalyst, particularly when the engine is returned to normal running and before the catalyst has had an opportunity to return to its operating temperature.
It is, therefore, a still further object of this invention to provide an improved arrangement for engine braking in a two-cycle, crankcase compression diesel engine wherein the amount of air pumped through the exhaust system under engine braking conditions is substantially reduced and the temperature of the air being pumped is increased so as to avoid quenching of the catalyst and exhaust system.
With diesel engines there are also additional considerations of the desirability of reducing the amount of smoke in the exhaust. It has been found that the amount of smoke under a wide variety of running conditions can be controlled appropriately through the use of scavenge and/or exhaust control.
It is, therefore, a still further object of this invention to provide an improved arrangement for controlling the smoke in the exhaust of a diesel engine through control of the scavenge and/or exhaust system.